Engines for vehicles, e.g. motor vehicles, often comprise a turbocharger comprising a compressor configured to increase the pressure of inlet air entering the engine, and hence, the amount of fuel that may be combusted within the engine to provide torque to drive the vehicle. The compressor is driven by an exhaust turbine. The turbocharger responds to requests from a driver of the vehicle for increased torque to be supplied by the engine. A boost control loop controls the turbocharger to provide the required boost in the intake manifold, thereby controlling the mass of gas entering the engine. In this way, the boost loop improves engine performance and a driving experience of the driver.
Engines are also often provided with Exhaust Gas Recirculation (EGR) systems configured to recirculate a portion of the burnt exhaust gases to an inlet of the engine. Replacing a portion of the oxygen rich inlet air with burnt exhaust gases reduces the proportion of the contents of each cylinder that is available for combustion. This results in a lower heat release and lower peak cylinder temperature and thereby reduces the formation of NOX, improving the emissions performance of the vehicle. An EGR control loop of the engine controls flow of exhaust gases from the exhaust manifold to the intake manifold. In boosted engines, the EGR system may include a high pressure EGR line for recirculating exhaust gases from upstream of the turbine to downstream of the compressor, and a low pressure EGR line for recirculating exhaust gases from downstream of the turbine to upstream of the compressor.
The operation of the EGR and turbocharger systems is interrelated and hence careful control of both systems is required to provide the driver with a good response to a request for increased torque from the engine, whilst maintaining good emissions performance. In case an excess volume of exhaust gases is recirculated, the torque response of the engine may be poor, which may affect a driving experience. On the other hand, if insufficient amount of exhaust gases are recirculated, NOx emissions are increased. Further, with more stringent emissions control regulations, great reliance is placed on EGR for controlling emissions, while at the same time, customer demand for quick responding engines is also increasing. Such competing and conflicting requirements often result in compromising on one of the two gas control loops (viz. the boost loop and the EGR loop).
As an example, when the driver requests increased torque to be supplied from the engine, e.g. by pressing an accelerator pedal of the vehicle (also referred to as a tip-in), the operation of the turbocharger is controlled to increase boost pressure as quickly as possible. For example, vanes of the turbine may be moved to a more closed position in order to increase the inlet pressure to the engine. However, quick closing of the vanes results in an increase in exhaust manifold pressure, which in turn causes an increase in pressure drop across the EGR line. Further, increase in the pressure drop leads to an increase in an EGR flow, if a position of an EGR valve of the EGR line remains constant. Consequently, an amount of fresh charge in the intake manifold may reduce. The engine may therefore provide an inadequate response to the request for increased torque. Thus, instead of having an increased torque production, torque production may be decreased, which may negatively impact the boost loop and thus, the driving experience.
One example approach for controlling boost pressure in an internal combustion engine is described in EP patent application 1178192 ('192 application). Therein vanes of a turbine of the engine are adjusted to control boost pressure in an intake passage. In particular, the vanes are adjusted based on engine operating conditions such as engine speed, fuel and oil consumption, water temperature, boost pressure, atmospheric pressure, atmospheric temperature, and a position of the EGR valve.
However, the inventors herein have recognized potential issues with such systems. As one example, the '192 application describes vane adjustments for controlling a pressure downstream of the turbine, based on multiple parameters. As such, the approach of the 192 application may not be able to control a pressure upstream of the turbine. As another example, the '192 application provides a static mechanism for controlling the vane positions, which may not adequately optimize the EGR loop and the boost loop in cases where fast response may be required.
In one example, the issues described above may be addressed by a method for a turbocharged engine comprising: estimating a maximum allowable rate of increase of exhaust manifold (EM) pressure based on a position of an exhaust gas recirculation (EGR) valve; estimating a maximum allowable EM pressure based on the maximum allowable rate of increase of EM pressure; and adjusting a vane position of an exhaust turbine based on the maximum allowable EM pressure. In this way, EGR and boost pressure control may be better coordinated for a fast torque response. In particular, the method enables maintaining of an exhaust manifold (EM) pressure of an engine.
As one example implementation, a turbocharged engine may be operating with high pressure EGR and boost enabled. At each time step of boosted engine operation, or responsive to an operator torque demand, an engine controller may determine an exhaust manifold (EM) pressure, that is, a pressure of exhaust gases upstream of a turbine of the turbocharger. The pressure of exhaust gases upstream of the turbine may be determined by referring to a data model or look-up table.
The controller may further determine a maximum permitted rate of increase of EM pressure based on a position of an exhaust gas recirculation (EGR) valve. The maximum permitted rate of increase may correspond to a maximum allowable rate for a given position of the EGR valve. Further, at each time step, a maximum allowable EM pressure may be determined based on the rate of increase of the EM pressure. The maximum allowable pressure is indicative of an EM pressure to maintain a mass flow of recirculated exhaust gas within a permissible limit. In one example, the maximum permitted value of pressure may be determined by multiplying the maximum permitted rate of increase in pressure by a particular period of time, for example, the length of the time step over which the method is performed, to calculate a maximum permitted increase in pressure; and then adding the maximum permitted increase to the determined pressure of exhaust gases upstream of the turbine.
Then, based on the maximum allowable EM pressure, at the each time step, operation of the turbocharger may be controller at least partially according to the pressure of the exhaust gases upstream of the turbine, such that the rate of increase in pressure of the exhaust gases is maintained at or below the maximum permitted rate. In one example, the turbocharger may comprise a Variable Geometry Turbine (VGT), wherein operation of the turbocharger assembly may be controlled by varying the geometry of the VGT. For example, a position of a vane of a turbine of the internal combustion engine may be controlled so that the EM pressure is maintained at or below the maximum allowable EM pressure and/or the maximum permitted rate of increase in EM pressure. Additionally or alternatively, the turbocharger may comprise a turbocharger assembly bypass duct, configured to permit exhaust gases to bypass a turbocharger assembly of the turbocharger assembly. For example, the bypass duct may allow exhaust gases to bypass the turbine of the turbocharger assembly. Therein, operation of the turbocharger assembly may be controlled by varying the flow of exhaust gases through the bypass duct by varying a position of a bypass valve in the bypass duct. Herein, the maximum allowable EM pressure and/or the maximum permitted rate of increase in EM pressure may be determined at least partially according to the position of the EGR valve. The controller may further determine a mass flow rate of exhaust gases through the turbine. The operation of the turbocharger may be controlled at least partially according to the mass flow rate of exhaust gases through the turbine.
As another example, the controller may control a degree of closing and/or a speed of closing of a vane of the turbine, based on the maximum allowable EM pressure, to optimize the EGR loop and the boost loop of the internal combustion engine. For example, the speed of closing the vanes of the turbines may be controlled by controlling vane positions.
In one example, to control the turbine vane position, a position of the vane may be determined based on an inverted turbine model. The inverted turbine model may provide the vane position as a function of the maximum allowable EM pressure, exhaust pressure downstream the turbine, and a rate of mass flow of the exhaust gases in the turbine. Further, the determined vane position may be compared with a default value of the vane position. The default value refers to a value of the vane position which is determined independent of the maximum allowable pressure. Based on the comparison, a minimum of the two values may be ascertained and used as a final value of the vane position.
The method may be performed iteratively over a plurality of time steps, as an example. Each step of the method may be performed during each time step. Alternatively, one or more of the steps may be omitted when being performed within the iterative process. Iteration of the steps of the method may be continued until a predetermined period of time has elapsed. In another example, the controller may detect a request for an increased amount of torque to be supplied by the engine, e.g. from a driver or controller of the vehicle. The method may be performed iteratively over the plurality of time steps responsive to detection of the request for increased torque.
The controller may further vary the position of the EGR valve such that the flow rate of exhaust gases within the EGR duct remains substantially constant. The operation of the turbocharger may be controlled such that the position of the EGR valve may be varied to maintain the flow rate of exhaust gases within the EGR duct at a substantially constant value.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.